In steel sheet to be worked, it is generally required to establish workability and strength with a good balance and minimize aging in order to avoid occurrence of stretcher strain that degrades the surface property of the product after forming.
On the other hand, it is preferable to enable cost reduction from the aspect of steel sheet production and enable annealing at low temperature from the aspect of productivity, but thin material tends to experience steel sheet buckling, called heat buckling, in the continuous annealing process during sheet production, and to avoid this, annealing must be made possible at a low temperature with a low recrystallization temperature. Particularly in the case where the sheet width of the passed coil is wide, heat buckling readily occurs owing to the difficulty of uniform external force control across the whole sheet width, so that in a very thin material the inability to provide wide coils has been a perpetual issue notwithstanding the need for wide coils from the viewpoint of productivity improvement during utilization by the steel sheet user.
For improving workability and suppressing stretcher strain, Patent Documents 1 to 6 listed below set out techniques for anti-aging by lowering C and N content and further adding Ti, Nb, B and other carbonitride-forming elements. However, in the thin material to which the present invention is directed, their use is limited from the viewpoint of heat buckling, because these elements greatly increase the recrystallization temperature of the steel sheet. Further, with heavy addition, the impact of alloying cost cannot be avoided, and health problems are also a concern in food-related materials.
Further, Patent Document 7 discloses can steel sheet of reduced C content that is excellent in deep drawability and earing property. In addition, stock sheet for surface treatment and can-making steel sheet of reduced N and Al content are disclosed in Patent Document 8, which is aimed at achieving fine precipitation of TiN and NbC in order to prevent surface roughness, and in Patent Document 9, which is aimed at lowering iron ion elution from the steel sheet surface. Further, Patent Document 10 teaches a method of producing can-making steel sheet of reduced C and N content that is aimed at lowering production cost.
However, materials of reduced C and N content such as set out in the aforesaid Patent Documents 1 to 10 have reduced strength, so that in a thin material that is the object of the present invention, a concern of ensuring container strength arises, and when Mn, Si, P or other strengthening elements are added to establish strength, surface property issues arise regarding platability, corrosion resistance and the like. Further, although a method of re-cold-rolling after annealing has been implemented as a method for strengthening without addition of strengthening elements, a marked decline in workability cannot be avoided.
Further, although container manufacturing processes frequently use welding to form the container itself or a handle or the like thereof, the weld strength of a material of low C and N content is often insufficient at the structural change in the steel cooling process. Further, as a method for easily measuring weld pass/fail at the welding site, a test, known as the Hyne Test, is performed in which the weld seam is pulled to tear the weld at the weld heat-affected zone and the state of the weld seam at this time is investigated, but if the weld seam is too soft at this time, the weld seam breaks to make normal testing impossible, thereby not only hindering the determination of suitable welding conditions but also making it impossible to select a material having good weldability. Further, when the C and N contents are low, the crystal structure at the heat-affected zone during welding becomes coarse and soft, so that strain concentrates in the heat-affected zone softened during processing of the weld, thereby degrading workability.
Further, in the course of producing a low C and N steel, carburization and nitrogen absorption may occur, depending on the production conditions, to vary the material properties in the coil or the production lot. Depending on the amount of added Ti, Nb and the like, the form and amount of the precipitates readily vary with the heat history of the production process, and this may also cause uneven material properties in the coil.
In other words, in these conventional technologies, steel sheet has not been obtained that is, on an elevated level, satisfactory in properties such as strength and workability, anti-aging property and platability, and to as far as heat buckling and alloying cost, plus weld zone properties, as well as productivity and production cost with attention to material handling ease during welding.